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<title>CS 537 - Programming Assignment I</title>
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<h1>
CS 537<br>Programming Assignment I
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<h2> Due:  </h2> September 17 at the <strong>start</strong> of class


<hr>
<h2>Contents</h2>
<ul>
<li> <!WA0><a href="#intro">        Introduction</a>
<li> <!WA1><a href="#suggestions">Suggestions</a>
<li> <!WA2><a href="#scanning">    Scanning</a>
<li> <!WA3><a href="#commands">    Commands</a>
<li> <!WA4><a href="#threads">    Using Threads</a>
<li> <!WA5><a href="#exceptions">    Exceptions</a>
<li> <!WA6><a href="#grading">    Grading</a>
<li> <!WA7><a href="#misc">        Other Stuff</a>
</ul>

<hr>

<a name="intro"> <h2>    Introduction </h2> </a>
<p>
The purpose of this assignment is to introduce you to Java programming.  
You are to implement a simple shell (command interpreter) that behaves 
similarly to the UNIX shell. When you type in a command (in response to 
its prompt), it will create a thread that will execute the command you 
entered.  Multiple commands can be chained together on a single line,
separated by `&amp;' (ampersand) characters.  Your shell will create a
thread for each individual command and prompt for more
user input when they have all finished.
<p>
Unlike the real shell, your program only has to deal with a handful of
``built-in'' commands:
<table align=center width="80%">
<tr>
    <td> <b>cat</b> <i>file ...</i>
    <td> Print the contents of the named files to <samp>System.out</samp> one
    after the other.
<tr>
    <td> <b>cmp</b> <i>file1 file2</i>
    <td> Check whether the two files have identical contents and print an
    appropriate message to <samp>System.out</samp>.
<tr>
    <td> <b>sort</b> <i>file</i>
    <td> Print the lines of <i>file</i> in sorted (lexicographic) order.
<tr>
    <td> <b>exit</b>
    <td> Terminate the program.  You should also terminate on reaching
    end-of-file on <samp>System.in</samp>.
</table>
You needn't implement 
pipes or re-direction of standard input and standard output, but you 
must be able to handle an arbitrary number of commands per line -- 
each with an arbitrary number of arguments separated by arbitrary 
amounts of white space (blanks or tabs) (although only <em>cat</em>
takes more than two arguments).

<a name="suggestions"> <h2> Suggestions </h2> </a>

Your <samp>public static void main()</samp> procedure in your primary class
will be quite simple.  It will be an infinite loop that prints a prompt, reads
a line (in other courses, a program with an infinite loop is considered a bad
thing, but in Operating systems, it's the norm!),
parses it (breaks it up into its constituent commands),
starts a new thread to handle each of the different commands,
and then waits for all the threads to finish before printing the next prompt.

<a name="scanning"> <h2>Scanning </h2> </a>

For scanning, you may find it easier to read the entire line into a String
object.  The
<!WA8><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.lang.System.html#10726">
<B>System.in</B> </a>
object is of type
<!WA9><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.io.InputStream.html">
<B>InputStream</B>, </a>
so it can read either single bytes or arrays of bytes.
You could represent an input line as an array of bytes, but you will find
it much easier to use a
<!WA10><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.lang.String.html">
<B>String</B> </a>
instead.  You may want to look into the class
<!WA11><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.io.DataInputStream.html">
<B>DataInputStream</B> </a>
to figure out how to read a line into a String.
Tokenizing a String is made almost trivial using the
<!WA12><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.util.StringTokenizer.html">
<B>StringTokenizer</B> </a>
class found in
<!WA13><a href="http://www.cs.wisc.edu/~cs537-1/java/api/javae.htm">
<I>java.util</I> </a>.

<a name="commands"> <h2>Commands </h2> </a>

For the <B>cat</B> command, you should look at the class <!WA14><a
href="http://www.cs.wisc.edu/~cs537-1/java/api/java.io.FileInputStream.html"> <B>FileInputStream</B>
</a> to see how to read data from a file.  <B>cmp</B> will be similar to cat,
but this time the contents of the two files will be compared.  For <B>sort</B>,
an efficient sorting algorithm is not required; anything that works is ok.
You might want to adapt the insertion sort used to introduce Java in
the discussion sections.
Some of the classes that might help you here are the <!WA15><a
href="http://www.cs.wisc.edu/~cs537-1/java/api/java.io.DataInputStream.html"> <B>DataInputStream</B>
</a> class and the <!WA16><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.util.Vector.html">
<B>Vector</B> </a> class.

<a name="threads"> <h2>Using Threads </h2> </a>

Your primary class will read a command from a user and then will create a
thread to carry out the command.  It will then wait until the thread has
finished before continuing its own execution.  There are two ways to start
threads in Java.  The first is to derive your class from the <!WA17><a
href="http://www.cs.wisc.edu/~cs537-1/java/api/java.lang.Thread.html"> <B>Thread</B> </a> class and
then override its <samp>run()</samp> function (see pp. 161-162 in <!WA18><a
href="http://www.cs.wisc.edu/~cs537-1/cs537.html#java-book"> the text</a>).  The second is to use the <!WA19><a
href="http://www.cs.wisc.edu/~cs537-1/java/api/java.lang.Runnable.html#42925"> <B>Runnable</B> </a>
interface (pp. 177-178).  Here you create a class that implements this
interface.  You then pass a reference to this class into the constructor of a
new thread object.  The former is perhaps easier to understand
conceptually but the latter is more general.  You may choose
either method for this assignment.

<a name="exceptions"> <h2>Exceptions </h2> </a>

Java requires you to place within a <B>try</B> block any methods that might
cause an exception.  Following the try block is a <B>catch</B> clause
(or catch clauses) that will
be used to catch any exceptions that have been thrown (see chapter 7 for more
details on the syntax of these statements).  Your code should deal with
exceptions in an appropriate manner.  For example, exceptions such as
attempting to open a file that does not exist should 
result in a message to the user and the continuation of the program.  
More serious exceptions may require an error message followed by program 
termination (using
<!WA20><a href="http://www.cs.wisc.edu/~cs537-1/java/api/java.lang.System.html#10832">
<samp>System.exit()</samp></a>).

<a name="grading"> <h2>Grading </h2> </a>

Hand in your source program and a transcript of a terminal session which demonstrates your shell's ability 
to perform as specified (see <samp>script(1)</samp>).  Be sure that you use test data adequate to exercise 
your program's capabilities.  You should follow all the principles of software engineering you learned in 
CS 302 and CS 367, including top-down design, good indentation, meaningful variable names, modularity, 
and helpful comments.  You will be graded not only on the basis of correctness, but also programming
style and completeness of test data.

<a name="misc"> <h2>    Other Stuff </h2> </a>

For those of you writing your programs on the Solaris machines, you might consider using
a makefile to aid in compilation.  Those of you using DOS may want to look into <B>doskey</B>, 
a little program that provides for easy command manipulation.  Feel free to send any other
work-saving methods you find to
<!WA21><A HREF="mailto:mellen@cs.wisc.edu">
the ta</a>, and he will distribute them to the class.

<hr>

<address>
<i>
<!WA22><a HREF="mailto:solomon@cs.wisc.edu">
solomon@cs.wisc.edu
</a>
<br>
Thu Oct 31 15:38:52 CST 1996
</i>
</address>
<br>
Copyright &#169; 1996 by Marvin Solomon.  All rights reserved.

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